Hi Patrick,
I also have a small shop. I also have lower back pain that makes standing for more than 45 minutes uncomfortable.
I came up with the cantilevered vise that tucks away under the bench when not in use. It also allows me to sit and work.
It takes a lot of weight to keep a portable vise still and the higher it is the more wiggle. The column is bolted to a cast iron machinists plate that weighs about 300 pounds. It wasn’t enough, so I put 225 pounds of bullet lead on top (in the green box) If I had it to do over I would just make the column longer and bolt it to the floor.
Sleeving it vertical it like Bvas suggest is a good idea. [attachment=1